This tool quickly calculates the required positions of your frets. All you have to do is enter your scale length**.
You can enter your scale in mm of in inches, it doesn't change the calculation. The result will be in the same mm or inches as you entered.
The 23rd and 24th frets are optional. If you want to have a reach of two octaves, use them. If you don't want that, than don't. It does not change the positions of the other frets as the scale length remains the same.
**The length of your scale is the distance between your nut, measured from the side against your fretboard, and your bridge, measured from the place where the string hits the saddle. Common scales are:
Gibson: varies from 62.38, 62.55 & 62.87 cm and 64.26 cm for a long scale
Fender strat & tele: 64.77cm
Fender Jaguar: 60.96 cm
Fender Mustang & Duosonic: 57.15 cm
Martin standard: 64.36 cm
Martin short scale: 63,09 cm
PRS: 63.5 cm
Classical guitar short scale: 65 cm
Classical guitar long scale: 66 cm
Frequently used for basses is the Fender scale:
Long scale: 86.36 cm
Short scale 76.2 cm
When you're placing your bridge at a later stage and if you have movable saddles, it makes sense to position your bridge in such a way that your scale length is right in the middle of the outer sides to which your saddles can be moved.